fix(guest): serialize concurrent queue-mutation playback per bot

The queue-mutating playback routes (play-now-song, play-next-song,
add-song, play-at) read queue position synchronously, mutate the queue,
then await resolveAndPlay() which suspends at an async URL fetch before
player.play(). With no serialization, two concurrent requests (normal in
login-less guest mode) interleave: the audible song (decided by URL-fetch
latency) can disagree with queue.currentIndex (decided by sync-block
ordering), corrupting "now playing" and causing skipped/duplicate songs.

Add a per-bot async serializer (BotInstance.runExclusive) and wrap the
critical region of all four routes in it. Single-request behavior and
every response shape / validation 400 are preserved; only the critical
region moved inside runExclusive.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/bot/instance.test.ts

@@ -0,0 +1,112 @@
+import { describe, it, expect } from "vitest";
+import { BotInstance } from "./instance.js";
+
+// Constructing a real BotInstance is heavy (spawns a TS3Client, AudioPlayer,
+// reads avatars, etc.), and runExclusive only touches a single private field
+// (`playGate`). So we exercise the ACTUAL shipped method via its prototype,
+// bound to a minimal object carrying just that field. This proves the real
+// serializer logic without standing up a full bot.
+type Gate = { playGate: Promise<unknown> };
+const runExclusive = BotInstance.prototype.runExclusive as <T>(
+  this: Gate,
+  fn: () => Promise<T>,
+) => Promise<T>;
+
+function makeGate(): Gate {
+  return { playGate: Promise.resolve() };
+}
+
+/** An explicit, timer-free deferred so ordering is deterministic. */
+function deferred<T = void>() {
+  let resolve!: (value: T) => void;
+  let reject!: (reason?: unknown) => void;
+  const promise = new Promise<T>((res, rej) => {
+    resolve = res;
+    reject = rej;
+  });
+  return { promise, resolve, reject };
+}
+
+describe("BotInstance.runExclusive — serialization", () => {
+  it("does not start fnB until fnA settles", async () => {
+    const gate = makeGate();
+    const order: string[] = [];
+    const gateA = deferred();
+
+    const pA = runExclusive.call(gate, async () => {
+      order.push("A-start");
+      await gateA.promise; // suspend A until we explicitly release it
+      order.push("A-end");
+    });
+
+    const pB = runExclusive.call(gate, async () => {
+      order.push("B-start");
+      order.push("B-end");
+    });
+
+    // Give the microtask queue a chance: B must NOT have started while A is
+    // still suspended on gateA.
+    await Promise.resolve();
+    await Promise.resolve();
+    expect(order).toEqual(["A-start"]);
+
+    gateA.resolve();
+    await pA;
+    await pB;
+
+    expect(order).toEqual(["A-start", "A-end", "B-start", "B-end"]);
+  });
+
+  it("runs fnB even if fnA rejects (chain survives rejection)", async () => {
+    const gate = makeGate();
+    const order: string[] = [];
+    const gateA = deferred();
+
+    const pA = runExclusive.call(gate, async () => {
+      order.push("A-start");
+      await gateA.promise;
+      throw new Error("A blew up");
+    });
+
+    const pB = runExclusive.call(gate, async () => {
+      order.push("B-start");
+      order.push("B-end");
+      return "B-result";
+    });
+
+    await Promise.resolve();
+    await Promise.resolve();
+    expect(order).toEqual(["A-start"]);
+
+    gateA.reject(new Error("A blew up"));
+    await expect(pA).rejects.toThrow("A blew up");
+
+    // B still runs, only after A has fully settled.
+    await expect(pB).resolves.toBe("B-result");
+    expect(order).toEqual(["A-start", "B-start", "B-end"]);
+  });
+
+  it("preserves call order across three serialized tasks", async () => {
+    const gate = makeGate();
+    const order: string[] = [];
+    const tasks = ["X", "Y", "Z"];
+    const promises = tasks.map((t) =>
+      runExclusive.call(gate, async () => {
+        order.push(`${t}-start`);
+        await Promise.resolve();
+        order.push(`${t}-end`);
+      }),
+    );
+
+    await Promise.all(promises);
+
+    expect(order).toEqual([
+      "X-start",
+      "X-end",
+      "Y-start",
+      "Y-end",
+      "Z-start",
+      "Z-end",
+    ]);
+  });
+});